The continuing goal of this project is to define the oncogenic effects of ionizing radiation on the mammary glands at the cellular and subcellular levels in vivo. Toward this goal we have sought to fulfill two long-term aims in a quantitative rat mammary cancer induction model. First, we set about to estimate the frequency of neoplastic initiation, the first step in carcinogenesis, per cancer-susceptible mammary cell in vivo. Second, we have endeavored to identify and concentrate the cancer susceptible cells and to characterize their morphology. physiology and response to ionizing radiation exposure. We hypothesized that the mammary cancer progenitor cells are members of a relatively undifferentiated epithelial cell subpopulation with high capacities for proliferation and self renewal and for giving rise to daughter cells that differentiate. We developed quantitative transplantation and hormonal stimulation procedures to determine the concentrations and numbers and to define the characteristics of these cells termed "clonogens", and have shown that mammary cancer develops at a very high incidence within the clonal glandular structures to which irradiated clonogens give rise when transplanted and hormonally stimulated. We now hypothesize that the recently shown very high sensitivity of the breast tissue of prepubertal and pubertal girls to radiogenic initiation is a result of interactions between the concentrations and physiologic conditions of the mammary clonogens and the internal milieu of sexually immature individuals, and believe it can be modeled in our quantitative rat mammary cancer induction system. We have shown that mammary clonogens from prepubertal rats are more sensitive to radiogenic killing than those from postpubertal animals, and suggest that this is because of a less efficient repair of radiogenic damage than in clonogens of mature individuals. We have also made progress in concentration and characterization of mammary clonogens, and have shown that mammary cells can be induced to form multicellular gland-like structures in culture.